I too once encountered a precision problem - it's been a while, but I think it 
was either in the exp() or log() function. I pretty much nailed it down to 
that - when I put my snippet of code into a Pascal file it worked without 
fault, but in Excel/Visual Basic it just produced crap.

Peter


On Tuesday 06 March 2007 10:26, Tom Brenna wrote:
> Hello Tom,
>
> The statisticians in my unit routinely warn us about Excel calculations,
> and this goes beyond statistics even to graphics.  They cite several
> articles showing that Excel has problems, among which are:
>
> (1)
> <http://www.stat.uni-muenchen.de/%7Eknuesel/elv/excelxp.pdf>http://www.stat
>.uni-muenchen.de/~knuesel/elv/excelxp.pdf
>
> (2)  D. McCullough B.; Wilson B.  On the accuracy of statistical procedures
> in Microsoft Excel 2000 and Excel XP.
> <http://www.ingentaconnect.com/content/els/01679473;jsessionid=3nn4j0onoc27
>1.victoria>Computational Statistics and Data Analysis, Volume 40, Number 4,
> 28 October 2002, pp. 713-721(9).
>
> (3)
> <http://gcrc.ucsd.edu/biostatistics/Excel.pdf>http://gcrc.ucsd.edu/biostati
>stics/Excel.pdf
>
>
> This being said, I use Excel all the time for calculations and for graphs,
> and suppose I will eventually be sorry.
>
> Tom Brenna
>
> At 10:56 AM 3/3/2007, you wrote:
> >Hello All,
> >We've been asked recently to provide results for a project in atom percent
> >instead of the usual delta per mil notation. It took a little while to
> >find the equations for this, so I decided to add one more place online
> >where this could be done. What I found in the process is that if I convert
> >delta per mil to atom percent using Excel, I get one result. If I use my
> >old TI-30, I get a slightly different result. If I use php
> >(http://localhost/uasil/black/isotopes/delta_to_atom_percent.php), I get
> >the same result as with the TI-30. The results of the calculator and php
> >are different from what I get with Excel starting with the 3rd to 5th
> >decimal place, depending on the standard and isotope used. This degree of
> >precision is normally adequate, but I'd expect better; particularly if I'm
> >making a number of calculations. I'm assuming all three tools have
> >adequate floating point precision to produce similar results for more than
> >just a couple of decimal places.
> >Why would there be differences after the 2nd decimal place with only a few
> >calculations (in the case of delta per mil to atom percent)? I'm a little
> >curious (concerned) as we use Excel daily for much more complicated
> >calculations.
> >Thanks,
> >-Tom
> >
> >~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >University of Arkansas Stable Isotope Laboratory
> >http://www.uark.edu/ua/isotope/

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